In office tables, peripheral devices for computers, copying machines, measuring instruments, telecommunication equipments, medical instruments, various office equipments, and the like, slide portions are utilized by over-stroking them over entire lengths of rails.
A slide unit for linear motion is utilized as a mechanical element for smoothly and precisely performing such sliding operation of the slide portion.
FIG. 4 represents a typical one example of a slide unit 50 for the linear motion of a conventional structure.
The linear motion slide unit 50 is generally composed of a pair of inner rails 60, 60 secured back to back to each other, a pair of outer rails 70, 70 attached to the inner rails 60, 60 respectively to be axially reciprocal, a number of balls 80 interposed between corresponding inner and outer rails 60 and 70 in rolling contact to the rails, and a retainer 90 for holding the balls 80 in a predetermined positional relationship.
The inner and outer rails 60 and 70 are formed of steel plates subjected to a precise roll-forming process, and these rails 60 and 70 are formed with ball rolling grooves 61, 61 and 71, 71 on upper and lower surfaces thereof so as to oppose to each other.
The retainer 90 is formed with a number of holes 91 corresponding in numbers to the balls 80 and, hence, since the balls 80 are arranged with equal spaces with each other always, respectively, the balls 80 are free from the mutual friction, thus obtaining a smooth sliding mechanism.
Then, a case where such linear motion slide unit 50 is applied to a slide portion of a drawer of, for example, an office table will be described hereunder.
One of the paired outer rails 70 is fastened to a body 110 of the office table by means of bolt or the like and the other one of the rails 70 is fastened to a side surface of the drawer 100 of the office table by means of bolt or the like. A possible extending length of the drawer 100 is determined to be equal to an over-stroke amount of the other one of the outer rails 70 with respect to one of the outer rails 70. As shown in FIG. 5(a), it is assumed that the linear motion slide unit 50 be composed of a portion X.sub.1 in which a number of balls 80 are disposed and both side slidable portions X.sub.2. As shown in FIG. 5(b), when the drawer is fully drawn out, the front end of the other one of the outer rails 70 can be drawn out from the front end of one of the outer rails 70 by an amount four times of the length of the portion X.sub.2. Accordingly, the over-stroke amount of the linear motion slide unit 50 is made long as the length of the portion X.sub.2 is possibly extended, and in other words, as the length of the portion X.sub.1, is possibly made short.
In the slide unit 50 for the linear motion, since the balls 80 are held always with equal space between adjacent ones by the retainer 90 formed by a precise pressing operation, the balls are free from the mutual friction and can be rolled smoothly.
However, in recent years, there has been required to utilize such kind of linear motion slide unit for a sliding portion on which a heavy load is applied. The described linear motion slide unit 50 is not suitable for the heavy load, and therefore, development of a linear motion slide unit capable of withstanding the heavy-load has been required.
In the case of requiring an increased over-stroke amount, as described above, the length of the portion X.sub.1, will be shorten. However, in the conventional linear motion slide unit, less number of balls 80 are utilized or less amount of a preload is applied in the present technology in this art, so that there is a limit for making short the length of the portion X.sub.1.